1. Field of the Invention
The present invention relates to an infrared absorbing compound and an optical recording medium using the compound and having excellent resistance to reproduction photo-deterioration and preservation stability.
2. Description of the Related Art
An optical recording medium such as an optical disk or the like has a substrate having spiral, circular or linear grooves. On the substrate is provided a recording layer, on which information can be recorded with high density by forming optically detectable small pits of, for example, the diameter off the pit is about 1 .mu.m, therein.
For example, when a laser beam converging on the surface of the recording layer scans the recording layer, the recording layer absorbs laser energy to form optically detectable pits thereon so that information can be written thereon. For example, in a heat mode recording system, the recording layer absorbs heat energy to form concave pits at absorption positions due to evaporation, decomposition or the like, so that information is recorded thereon.
A metal thin film such as an aluminum deposited film or the like, a chalcogenide amorphous glass film such as a bismuth thin film, a tellurium oxide thin film, or a like film mainly consisting of an inorganic substance, has been proposed so far as the recording layer of such an optical recording medium. On the other hand, it has recently been proposed that an organic dye which exhibits changes in physical properties when exposed by light having a relatively long wavelength can be used in a recording layer.
For example, when a polymethine dye, an azulenium dye, a pyrylium dye or the like, which exhibits greater sensitivity to exposure by a laser beam, is used in an organic dye thin film, a light absorption reflecting a film exhibiting metallic luster (a reflectance of about 10 to 50%) is obtained. This film enables the formation of an optical recording medium which permits both laser recording and reflection reading. Particularly when a semiconductor laser having an oscillation wavelength of 650 to 900 nm is used as a laser light source with the film, the size and cost of the recording apparatus can both be reduced.
However, when an organic dye is used in recording layer, the organic dye tends to deteriorate after repeated irradiation with reproduction light, and the reproduction properties of the optical recording medium thereby also deteriorate. A known method for solving such a problem involves the use of a metal chelate complex (particularly, an Ni chelate complex) as a singlet oxygen quencher. However, a metal chelate complex cannot be added in an amount which is sufficient to improve the light-resistance of an organic dye, because the chelate complex has a low solubility in solvents which can be coated on a plastic substrate.
In another method, which is proposed in U.S. Pat. No. 4,626,496, a double salt is formed by a metal complex and a dye, so that a larger amount of metal complex can be added to the recording layer. Although the light resistance of the organic dye can be improved with this method, the density of the organic dye in the recording layer is also decreased, thereby decreasing the recording sensitivity. Alternatively, an aminium salt or a diimonium salt compound that serves as a stabilizer as disclosed in U.S. Pat. Nos. 4,656,121 and 4,923,390, can be added to the recording layer in order to improve the light resistance of an organic dye. The method of adding an aminium salt or diimonium salt to the recording layer has the problem, however, that a large amount of such a salt must be added to improve the light resistance because the counter ion is an acid anion. In order to solve this problem, an example is disclosed in Japanese Patent Laid-Open No. 62-193891, in which a small amount of a double salt complex of an aminium salt cation or diimonium salt cation and a metal complex anion is added to improve the light resistance.
Although this method can improve the light resistance and recording reproduction properties, when the aminium salt cation or diimonium salt cation is substituted by an alkyl amino group, the double salt compound with a metal complex anion lacks sufficient solubility in the usual solvents. In particular, such a compound has low solubility in solvents such as aliphatic hydrocarbons, alcohols, ketones and the like, which can be directly applied to a plastic substrate. There is also the problem that an optical recording medium having a recording layer formed by a solvent coating method frequently has a high noise level.
In addition, Japanese Patent Laid-Open No. 3-164292 discloses an optical disk with excellent light-resistance, which comprises (1) a recording layer consisting of a mixture of a diimonium salt cation and a metal complex anion and (2) a reflecting layer formed on the recording layer. Examples of diimonium salt cations disclosed in the specification of 3-164292 include a diimonium salt of N,N,N',N'-tetrakis (dialkylamino-substituted phenyl)-p-phenylenediamine, a diimonium salt of N,N,N'N'-tetrakis (dialkoxyalkylamino-substituted phenyl)-p-phenylenediamine, and the like. However, an organic dye recording layer containing such a double salt compound has insufficient preservation stability under conditions of high temperature and high humidity. Also the degree of improvement in light resistance of the recording layer is unsatisfactory.